update gitlab to latest version (#343)

* update gitlab to latest version

This should refer to the offical docker compose file of gitlab: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/docker/docker-compose.yml

* added option to select gitlab external_url prefix

This is needed so that rancher will put a correct uri scheme prefix to the ${gitlab_hostname} to the external_url.

* added external_url prefix

honestly i don't know if `${gitlab_omnipus_prefix}${gitlab_hostname}` will work since i cannot test it by myself right now, but imo that should work....

* changed version to latest

since image is going to be gitlab/gitlab-ce:latest, the version here inside the readme should reflect this.

* update to latest

templates/gitlab/0/docker-compose.yml

@@ -6,9 +6,12 @@ gitlab-server:
   labels:
     io.rancher.sidekicks: gitlab-data
   hostname: ${gitlab_hostname}
-  image: gitlab/gitlab-ce:8.5.1-ce.0
+  image: gitlab/gitlab-ce:latest
   volumes_from:
   - gitlab-data
+  environment:
+    GITLAB_OMNIBUS_CONFIG: |
+      external_url '${gitlab_omnipus_prefix}${gitlab_hostname}'
 
 gitlab-data:
   labels:
@@ -16,7 +19,7 @@ gitlab-data:
   entrypoint:
   - /bin/true
   hostname: gitdata
-  image: gitlab/gitlab-ce:8.5.1-ce.0
+  image: gitlab/gitlab-ce:latest
   volumes:
   - /etc/gitlab
   - /var/log/gitlab